The invention relates to an electro-optical device for scanning an information tack on an information carrier. An objective having an optical axis is mounted in a holder which is movable relative to a frame plate so that the holder can be translated parallel to the optical axis and can be rotated about an axis of rotation which extends parallel to the optical axis. Bearing means support the holder relative to the frame plate and magnetic means are arranged on the frame plate surrounding the axis of rotation. At least two cylindrical wall portions are connected to the frame plate and surround said magnetic means in such a way that an annular air gap is formed between the wall portions and the magnetic means. The magnetic means and the cylindrical wall portions form part of a magnetic circuit with a radial magnetic field in the air gap. A focussing coil, which secured to the holder, which is movable in the air gap, and is arranged coaxially around the axis of rotation. At least two tracking coils are secured to the holder externally of the focusing coil and extend through recesses between said cylindrical wall portions. The turns of the tracking coils are situated in or substantially in radial planes containing the axis of rotation of the holder, the turns of each of the tracking coils together surrounding a continuous space. The focussing coil and the tracking coils, when energized, both cooperate with the magnetic circuit for controllably moving the objective.
Such a device is known from Japanese Patent Application No. 58-206142. The known device is intended for use in an optical-disc player or in another apparatus in which a light beam is focussed on the recording surface of an information carrier via an objective for the purpose of optically reading the information stored in a track on the surface. The objective is eccentrically mounted in a holder which is movable relative to the frame plate of the device. For this purpose the holder is provided with a centrally mounted bearing which is rotatable about and movable along a bearing pin secured in the frame plate. As the bearing pin extends parallel through the optical axis of the objective, focussing errors and tracking errors can be corrected by moving the holder along the bearing pin and rotating the holder about the bearing pin respectively. For driving the holder there are provided electromagnetic means, comprising coils arranged on the holder, an annular permanent magnet secured to the frame plate, and two cylindrically curved magnetically conductive wall portions arranged on the frame plate. A focussing coil is wound on a cylindrical wall of the holder to provide the axial movements of the holder and two rectangular tracking coils are cemented to the focussing coil to provide the pivotal movements of the holder. The turns of the focussing coil are coaxial with the bearing pin. The wall portions, which are diametrically spaced from each other, are each formed with a recess to provide a coil former having a free end, over which one of the tracking coils is slid. Between the free end of each coil former and the facing wall portion a free space is formed. Thus, between the magnet and the wall portions an annular air gap with a radial magnetic field is formed for cooperation with the coils, the air gap being interrupted at two locations and the radially directed magnetic field being discontinued at the location of the free spaces.
When the focussing coil is energized the holder can be moved axially for focussing the light beam on the recording surface of the information carrier. When the tracking coils are energized, which coils are movable relative to the coil formers, which extend through recesses in the cylindrical wall portions, and which may be situated partly in the free spaces, the holder can be pivoted about the bearing pin. This causes the objective to be moved substantially perpendicularly to the information track for the purpose of following the information track of the information carrier.
In the known device the magnetic circuit is asymmetrical at the location of the tracking coils. A disadvantage of this is that the flux in the coil formers is always oriented in one direction, which in the case of energizing of the tracking coils is likely to give rise to magnetic saturation, unless coil formers of comparatively large cross-section are used. This also means that large tracking coils are needed, which is undesirable for reasons of efficiency. Another disadvantage of the known device is the fairly large loss of magnetic flux at the free ends of the coil formers. A further disadvantage is that owing to the asymmetrical pattern of the magnetic lines of forces near the tracking coils the torque produced per unit of current through the tracking coils is variable, which results in a limited sensitivity of the device. Further, it is to be noted that the known construction, comprising the frame plate and the two separate cylindrical wall portions cannot readily be manufactured as a single part on account of the recesses between the coil formers and the frame plate. Therefore, the known construction is preferably manufactured by assembling the parts. However, accurately positioning these parts relative to each other is then a laborious procedure.